Likelihood Ratio Calculator

About this calculator

A diagnostic likelihood ratio tells you how much a test result should shift your belief about whether a patient has a condition. The positive likelihood ratio is LR+ = sensitivity / (1 − specificity), showing how much more likely a positive result is in someone with the disease versus without it; the negative likelihood ratio is LR− = (1 − sensitivity) / specificity, showing how much a negative result should lower suspicion. An LR+ above 10 or LR− below 0.1 is generally considered strong evidence.

Clinicians combine LR with a patient's pre-test probability (based on prevalence and clinical presentation) — often via a Fagan nomogram or Bayes' theorem — to compute the post-test probability of disease, making it central to evidence-based diagnostic reasoning. Epidemiologists use likelihood ratios to evaluate how useful a screening or diagnostic test actually is beyond its raw sensitivity and specificity, and medical students learn LR as a core tool for interpreting lab and imaging results in context.